Steam generating unit



g- 4, 1956 M. H. KUHNER 2,758,574

STEAM GENERATING UNIT Filed Feb. 24, 1953 4 Sheets-Sheet 1 oooooo oeo oaunoo coo INVENTOR 39 MAX H. KuH/vn? ATTORNEY Aug. 14, 1956 M; H. KUHNER STEAM GENERATING UNIT 4 Sheets-Sheet 3 Filed Feb. 24, 1953 a n a M a 0 R 2 2 v H )unnT-oaununuo-nf 5 w 5 N /o 2 lolnooro-\-vonouon kfl III I r mwH r L oo.nuouovuuuuoooul F 0 VA A h F N W.

INVENTOR Max H. KUHNER.

ATTORNEY United States Patent STEAM GENERATING UNIT Max H. Kuhner, Worcester, Mass., assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts Application February 24, 1953, Serial No. 338,433

Claims. (Cl. 122-240) This invention relates generally to steam generating units and more particularly to a furnace and boiler for use in providing power turbines with large quantities of steam at high temperature and pressure.

In the past it has been the practice in operating large steam generating units to control the temperatures of superheated and reheated steam either by use of by-pass apparatus, by shifting the zone of combustion up and down, or by recirculation of a portion of the gases. However, the prior art apparatus has many shortcomings, particularly when used in very large units such as generate over a million pounds of steam per hour. For one thing, considerable difliculty has been experienced with slag building up in the back passes of the boiler because the temperature of the gases in these passes is too high. Furthermore, although it is sometimes very desirable to have a considerable portion of the reheat surface exposed to the radiant heat of the furnace, it has not been possible to do so for many reasons, not the least of which is the ditficulty of starting up the unit with no cooling steam passing through radiant reheat tubes. Also, considerable difliculty has been experienced in controlling both superheat and reheat temperatures, due to the fact that the two heating surfaces have different load vs. heat-absorption characteristics and changing furnace conditions, for instance, brought about to produce a change in one may produce an undesirable effect on the other. These difiiculties have been obviated by the present invention in a novel manner.

It is, therefore, an outstanding object of the present invention to provide a steam generating unit which will permit the separate adjustment and control of superheat and reheat temperatures.

Another object of the invention is the provision of a steam generating unit having a radiant reheat section so constructed as to permit starting without difificulties due to lack of initial flow of steam through the reheater.

It is a further object of this invention to provide a watercooled furnace which affects even distribution of gas temperatures transversely and longitudinally leaving the furnace and entering the convection passes.

A still further object of the invention is the provision of means for maintaining the gases leaving the furnace and entering the convection passes at a temperature below the slagging temperature, the said means not involving an additionally large furnace or great expense.

Another object of the instant invention is the provision of a steam generating unit which is divided in a novel and practical manner into a plurality of water-cooled sections.

It is another object of the invention to provide a novel and inexpensive platen construction.

It is also an object of this invention to provide an arrangement of burners, superheater sections and reheater sections which will permit sensitive, reliable control over superheat and reheat at all loads.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For

a better understanding of the invention, the operating advantages and specific objects attained by its use, refer ence should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of my invention.

Figure 1 is a vertical, longitudinal sectional view of a steam generating unit embodying the principles of the present invention, the View being taken on the line 11 of Figure 3.

Figure 2 is a sectional view taken on the line 2-2 of Figure 3.

Figure 3 is a horizontal sectional view taken on the line 33 of Figure 1.

Figure 4 is a vertical sectional view taken on the line 4-4 of Figure 1.

Figure 5 is an elevational view of a portion of the unit, the view being taken along the line 55 of Figure 1.

Figure 6 is a graphical showing of the relationship of steam temperature to percentage of full load in the superheater of the present invention, and

Figure 7 is a graphical showing of the relationship of steam temperature to percentage of full load in the reheater of the invention.

Referring first to Figure 1, wherein is best shown the general features of the invention, the steam generating unit, designated by the reference character 10, is shown as comprising a furnace 11 and a boiler 12. The furnace consists of a front wall 13, a rear wall 14, and side walls 15 forming a vertically elongated combustion chamber 16. The boiler 12 consists in general of a steam-and-water drum 17, downcomer tubes 18, a riser system 19, primary superheater 20, secondary superheater 21, and reheater 22. (See Figure 2.) Extending rearwardly and generally horizontally from the upper edge of the front wall 13 of the furnace is a roof 23. Overlying this roof is an insulated chamber 24 in which lie the steam-and-water drum 17 and various headers as will be described hereinafter. The rear wall 14 of the furnace forms With a back wall 25 of the unit a back pass 26 extending vertically downwardly from the roof 23 and residing in juxtaposition to the combustion chamber 16. The lower end of the back pass 26 is connected to a duct 27 which is in turn connected to other apparatus, not shown, but which may be an air heater or the breaching of a chimney.

A feedwater inlet conduit 23 enters the lower part of the back pass 26 through the back wall 25 and is connected to a header 29 lying therein. The header is connected to the inlet of an economizer 30 of the usual type and a conduit 31 connects the outlet of the economizer to the steam-and-water drum 17. Conduits 32 connect the low part of the drum 17 to a transverse header 33 residing more or less in the plane of the back wall 25 of the unit. The header 33 is connected by downcomers 18, which are relatively large, to a transverse header 34 lying at the bottom of the combustion chamber 16. The header 34 is connected to a plurality of rear water-Wall tubes 35 forming a part of the riser system 1%. These tubes 35 extend upwardly along the rear wall 14 of the furnace in spaced, parallel relationship with one another. At a position substantially short of the roof 23, the tubes incline forwardly and upwardly to form a furnace nose 89, extending to a position midway between the front wall 13 and the rear wall 14 of the furnace, at which position they extend vertically and are connected at their upper ends to a header 36 lying inthe chamber 24 which is connected to the steam-and-water drum 17. Another series of tubes 37, which might be termed the front water-wall tubes, extend from the header 34 upwardly along the inner surface of the front wall 13 of the furnace; when these tubes reach the roof 23, they follow its under surface and extend rearwardly to'a position somewhat behind the steam-andwater drum 17, at which position they extend vertically 3. upwardly for connection at their upper ends to the said drum. The tubes 35 and 37 are inclined inwardly toward the header 34 at their lower ends and form a water-cooled, single-header' hopper 38 inthe lower portion of the combustion chamber 16. An ash discharge door 39 is provided in the forward wall of the furnace adjacent the apex of the hopper 38 and the tubes 37 are somewhat spaced in the area of this door to facilitate ash or slag removal.

A number or" forward vertical headers 40 are arranged transversely of the furnace outside the forward wall 13 proper; In the specific embodiment there are three such headers and from them extend platen tubes 41. These tubes 41 extend horizontally inwardly of the furnace a distance and then vertically in contact with one another to form a curtain or platen 42 which extends longitudinally of the furnace for half the distance from the forward wall '13 to the rearward wall 14. The tubes 41 making up the platen are part of the riser system 19 and extend upwardly to positions at the upper part of the combustion chamber from which they find their way to the steam-and-water drum 17. Vertical headers 43, similar to the headers 40, are situated outside the rearward wall 14 at positions exactly opposite the corresponding headers 40, and tubes 44 extend forwardly into the combustion chamber and then upwardly to form platen 45. The tubes, of course, find their way to the steam-and-water drum 17. The platens 45 extend longitudinally of the combustion chamber and meet the platens 42 centrally of the chamber. In other words, each set of platens 42 and 45 forma bare water-tube curtain extending longitudinally of the furnace, the lower edge of the curtain being situated about half-way up the combustion chamber and the curtain extending upwardly until it meets the roof 23.

Situated in the front wall 13 of the furnace are a series of burners, numbered 46 through 65, which are arranged in the manner shown in Figure 5. Hot air ducts 66 are situated at the sides of the furnace on a level intermediate of the burners and carry preheated air from an air heater or the like, not shown. The ducts 66 fan out adjacent the front of the furnace and the air is divided between ducts 67 leading to a secondary air chamber 69 serving the burners 62 through 65 and ducts 68 leading to a secondary air chamber 70 serving the burners 46 through 59. Control dampers 71 are situated at the junction of the ducts 66 and 67, while control dampers 72 are situated between the duct 66 and the duct 68, as is best shown in Figure 1.

Referring next to Figure 3, it can be seen that the curtains formed by the platens 42 and 45 divide the combustion chamber 16 into four compartments 73, '74, 75 and 76, in the upper portions of which are situated, respectively, portions of a section of the secondary superheater 21, a section of the reheater 22, a section of the reheater 22, and a section of the secondary superheater 21. To the rear of these members and lying in the back pass 26 is the primary superheater 20.

As is best shown in Figure 2, a reheat return line 77 feeds into a header 78 lying along the rearward wall 14 of the furnace. From this header extend radiant reheater tubes 79 which overlie the rearward wall of the furnace in the section lying in the compartments 74 and 75. The radiant reheater tubes extend upwardly, are spread out to form the screen 90, and are then connected to the convection reheater 12.

An examination of Figures 1, 2 and 3 will show the presence of a gas plenum chamber 80 in the space at the upper part of the back pass 26 and defined by the upper end of the primary superheater 20, the rear ends of the reheaters 22, and the rear ends of the high temperature superheaters 21. This is so that there will be uniform distribution of the combustion gas over the primary superheater regardless of the proportionate quantity of gas coming from the four furnace compartments.

Referring next to Figure 4, the primary superheater 20 resides centrally of the back pass 26 and by=pass passages 81 and 82 exist on either side. The passages are separated from the part of the back pass in which the superheater resides by walls 83 and 84, respectively, these walls extending well below the superheater. As is evident from the drawing, the economizer inlet header 29 is situated centrally of the back pass 26. The economizer extends completely across the back pass with the ends lying in the lower part of the passages 81 and 82 and it is provided with final sections 35. and 86 which reside only in the passages. Underlying the economizer and extending completely across the back pass 26 longitudinally thereof are gas flow control dampers some of which control gas flow through the lower end of the passages 81 and 82 and others of which do perform the same function for the central part of the back pass 26.

The operation of the steam generating unit will now be evident in view of the above description. Feedwater enters the apparatus through the inlet conduit 28 and enters the header 29 from which it is distributed to the various coils of the economizer 30. The heated feed water leaves the economizer and passes through the conduit 31- to the steam-and-water drum 17. The water then leaves the drum 17 through the conduits 32 to the header 33 from which it enters the downcomers 18 and passes downwardly into the header 34 from which it is distributed through the riser system 19. The water is converted to steam in the riser system and enters the steam-and-water drum after which the steam flows into the lower end of the primary superheater 20. After passing through the primary superheater, the steam passes through a desuperheater 88 from which it enters the downstream end of the secondary or high temperature superheater 21. The steam passes through the high temperature superheater and flows into the high-pressure section of a turbine, not shown, where it performs useful work and loses temperature and pressure. The exhaust from the high pressure section of the turbine is returned to the steam generating unit and re-entcrs through the reheat return line 77 into the header 78 from which it is distributed to the various radiant reheater tubes 79. The steam flows through the tubes 79 and eventually passes into the convection section of the reheater 22. The steam fiows through the convection section from rear to front of the furnace and is discharged into the low pressure section of the turbine.

Fuel flows to the burners 46 through 65 in the wellknown manner and heated secondary air is supplied from the ducts 66. The fuel flow to the burners can be controlled in the usual manner and the How of air from the ducts 66 into the chambers 60 and 70 can be controlled by use of the dampers 71 and 72'. The products of combustion from the burners pass upwardly through the combustion chamber 16 between the tube curtains formed by the platens 42 and 45 and in the chambers 73, 74, 75 and 76. The gas in the rearward part of the combustion chamber is guided forwardly by the nose 89 of the furnace and is mixed with the other gases before entering the upper pass 90 wherein reside the reheaters and the high-temperature superheater. The upper pass has substantially less height at the rear, so that the gas velocity is increased in that portion. After entering the plenum chamber and mixing thoroughly, the gas passes downwardly through the hack pass 26 and out through the duct 27.

Some of the advantages of the apparatus of the invention can be more easily understood in connection with a description of the application of principles of the invention to an actual steam generating unit. This is a unit having a capacity of 1,200,000 lbs. of steam per hour and is designed to burn oil, natural gas, or pulverized coal. The steam pressure at the superheater outlet is 1850 p. s. i. g. and the final steam temperature for both high pressure steam and reheat steam will be 1000 degrees F. This unit supplies steam for a 150,000 kw. turbine genamen erator. Maintenance of constant steam temperature over a wide range of load is important in this unit for the economic operation of the turbine. When the design temperature is as high as 1000 degrees F. it is vital that this temperature be not appreciably exceeded because the metals in the superheater, reheater, piping and turbine are selected for the service temperature. Prolonged exposure of these metals to higher temperatures increases their rate of creep and shortens their useful life. This unit is designed to maintain nearly constant steam temperature of 1000 degrees F. within the steaming range of 725,000 and 1,200,000 lbs. per hour when burning oil and from 600,000 to 1,200,000 lbs. per hour when burning natural gas. The essential elements of the steam temperature control system, according to the invention, are mainly gas flow control of high pressure steam and burner control for the reheat steam. These controls are supplemented by auxiliary desuperheater controls, both for. high pressure and reheat steam, but the desuperheater controls are secondary and are provided for emergency use only. The upper part of the furnace is divided into four compartments by tube platens consisting of 3% O. D. bare tubes on tangent spacing. The platens serve the principal purpose of providing suflicient water cooling area in the furnace to cool products of combustion so that ash carried in the combustion gas leaving the furnace is dry and will not stick to heating surfaces of the superheater, reheater and water screens. This is particu larly important in pulverized coal firing, but is also highly desirable for the oil firing because the fuel oils available today contain considerable quantities of impurities. Slagging troubles can become almost as serious when burning oil as with pulverized coal. With the upper sec tion of the furnace divided into four compartments it is practical to locate all reheater surfaces above the two center compartments and to place the surface of the high temperature section of the high pressure superheater above the two outside compartments. In this manner heat generating in the furnace is channeled to reheater surface and high temperature superheater surface separately and independently.

The boiler unit is equipped with twenty combination oil, gas and pulverized coal burners, all installed through the front wall of the furnace. There will be four vertical rows of five burners each. One each of the four vertical burner rows is directed into one of the four compartments but only the upper three burners of each row are enclosed by the compartment walls. The lower two burners discharge into the open furnace space below the platens. Six burners of the twenty are used exclusively for the control of the temperature of reheat steam. These are the three upper burners of each of the two center rows. These six burners are enclosed in a separate burner box which receives combustion air from the main air sup-- ply duct system through separate air connections. The burner group of six burners with its air and fuel supply system is entirely independent of the remaining fourteen burners and is controlled solely from reheat steam temperature through a separate automatic control system. The remaining fourteen burners are controlled in the usual manner through the automatic combustion control system which receives its impulse from steam flow and steam pressure.

The reheat temperature control burners are directed. into the'two center furnace compartments above which. the reheater surfaceis located. Higher reheat temperature is obtained by increasing the firing rate of these six burners. Conversely, the reheat temperature is reduced. with reduced firing rate. An automatic control system. which receives its impulse from the final reheat tempera-- ture adjusts the fuel flow and combustion air flow to these six burners so that the reheat temperature of 1000 degrees F. is kept constant over a wide range of load, as is shown in the graph of Figure 7.

Heat recovery per pound of reheat steam decreases with increasing load. For example, at a steam flow rate of 60% of rated capacity each pound of reheat steam must absorb 229 B. t. u.s to raise the temperature to 1000 degrees F whereas at steam flow rate each pound of reheat steam must absorb only 184.8 E. t. u.s to be heated to 1000 degrees F. In other words, 28% more heat per pound of steam must be supplied at a. steam flow rate of 725,000 lbs. per hour than at the rate of 1,200,000 lbs. per hour. For this reason a large portion of the reheater surface consists of radiant heat absorbing surface. The rear furnace wall within the two center compartments is covered by radiant heat absorbing reheater tubes which extend upwardly and form four rows of screen tubes between the furnace and the final reheater section. The low temperature reheat steam flows first through the radiant surface and then through the high temperature con-' vection section.

The performance characteristic of a radiant superheater is such that the steam temperature drops as the load increases. Full use is made of this characteristic in the design of the subject steam generating unit because heat recovery per pound of reheat steam is required to decrease as the steam flow increases. With the combination of radiant and convection heat absorbing surfaces, both placed in the center compartments of the furnace, nearly constant firing rate can be maintained by the six reheat control burners to maintain constant reheat temperature of 1000 degrees F. between approximately 60% and 100% of steam flow rates.

The high temperature superheater section is placed above the two outside compartments and the primary superheater section or low temperature superheater is placed across the rear of the boiler unit so that all combustion gas discharged from the four compartments will flow over the primary superheater surface. Two blank gas passages are provided along each side of the primary superheater. The vertical gas passage from the top of the boiler to the outlet of the economizer is partitioned so that there is one main gas pass over the primary superheater and one open gas pass on either side of the primary superheater. At the lower ends of these passages dampers are provided to proportion the gas flow between superheater pass and bypass, and in this manner to control the steam temperature. If the high pressure temperature has a tendency to rise above the 1000 degree F. limit, the dampers provided in the two bypasses will gradually open to permit part of the combustion gas to bypass the primary superheater surface. These dampers will continue to open as long as further down-control of the high pressure steam temperature is required. After the full open damper position has been reached, and, if still further bypassing is required, the dampers below the main pass will start to close, thus throttling the flow of gas over the primary superheater and forcing more gas through the bypass. The temperature is thus maintained substantially constant over a Wide range as indicated in Figure 6. The adjustment of gas flow control dampers is by automatic control which receives its impulse from the temperature of the high pressure steam. It is, of course, necessary to realize that the control of high pressure steam temperature by gas flow is not entirely independent of the control of the reheat temperature by burner adjustment. For example, if the burning rate of the group of six reheat burners is being reduced at high load, the remaining fourteen burners must take over the heat input dropped by the six reheat control burners. The high temperature section of the main superheater will then receive more heat, which calls for greater rate of bypassing. High pressure steam temperature control and reheat steam temperature control are therefore some what interdependent.

The starting-up period of reheat boilers is usually somewhat critical. The reheater is not cooled by steam until steam flows through the turbine. This means that there is a period during which pressure builds up in the boiler and while fuel is being burned in the furnace, when there is no cooling of the reheater surface. The pressure will have to reach 200 p. s. i. g. and more before steam can be fed to the turbine and before the turbine will start to roll. Only after steam is flowing through the turbine will steam return to the reheater and only then will there be cooling of reheater surfaces. The design of the subject unit permi-ts raising of pressure during starting up of the boiler without exposing the reheater surface to high temperature because during start-up only the two outside burners of the bottom row will be in service. The heat released by these burners will pass along the side walls of the furnace and the two center compartments containing the reheater will be completely bypassed. The primary superheater is drainable, so that it will be entirely free of condensate during starting up. The high temperature superheater section consists of vertical hanging loops which will be partly filled with condensate when the burners are first started. The combustion gas will pass over the high temperature section of the superheater only and the condensate will gradually evaporate. After boiler pressure of approximately 200 to 250 p. s. i. g. is reached steam will start to flow from boiler through superheater, through high pressure end of the turbine, through the reheater and back through the low pressure turbine to the condenser. It is then possible to light additional burners for increased steam flow and for rapid increase in pressure. It is anticipated that it will require approximately 3 to 4 hours of continuous firing using the two bottom outside burners to raise the pressure of the boiler from atmospheric to 200 p. s. i. g. and an additional 2 to 3 hours of firing with all bottom burners before full operating pressure and normal load are reached.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form of my invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit .of the invention covered by my claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A steam generating unit comprising a furnace having front, rear and side walls, a vertical header mounted outside the furnace adjacent a .wall, a series of water tubes connected to the header and passing through the said well into the furnace, the said water tubes extending upwardly within the furnace in parallel relationship to form .a first curtain extending into the furnace, a

second curtain similar to the first curtain, the curtains serving :to divide ;the furnace into compartments, a first group of burners mounted on the front wall at a position generally midway between the side walls and lying within a central compartment formed by the curtains, a isflqond group Eof burners mounted on the front wall and lying within a side compartment formed by a curtain adjacent one side wall, a third group of burners mounted on the front wall and lying within another side compartrnent formed by a ,curtain adjacent the other side wall, a fourth group of burners underlying the other groups and extending from side wall to side wall of the furnace, first means furnishing the first group of burners with secondary air, second means independent of the first means for Supplying the second, third and fourth groups of burners with secondary air, and means for controlling the rate. of firing of the first group independently of the second, third, and fourth groups to Qhtainexc gas te pe atu d s ibu n ac oss th upp nurtio nfthefu ae 2. A steam generating unit comprising a furnace hav- 8 1 ing front, rear and side walls, a first vertical header mounted 7 outside the furnace adjacent the front wall, a series of water tubes connected to the header and passing through the front wall into the furnace, the said water tubes'extending upwardly within the furnace in parallel relationship to form a first curtain extending from the front wall to a point midway between the front and rear wall, a second vertical header mounted outside and adjacent the rear wall, another series of water tubes connected to the second header and passing through the rear wall into the furnace, the latter series of tubes extending vertically and parallel to one another within the furnace to form a second curtain extending from the rear wall midway to the front wall, the first and second curtains lying in the same general vertical plane to form a first divider curtain, a second divider curtain similar to the first, the divider curtains serving to divide the furnace into compartments, a first group of burners mounted on the front wall generally midway between the side walls and lying within a central compartment formed by the curtains, a second group of burners mounted on the front wall adjacent one side wall and lying within a side compartment formed by a divider curtain, a third group of burners mounted on the front wall adjacent the other side wall and lying within another side compartment formed by a divider curtain, a fourth group of burners underlling the other groups and extending from side wall to side wall, and means for controlling the rate of firing of the first group independently of the second, third and fourth groups.

3. A steam generating unit comprising a furnace having from, rear and side walls, a first vertical header mounted outside the furnace adjacent the front wall, a series of water tubes connected to the header and passing through the front wall into the furnace, the said water tubes extending upwardly within the furnace in parallel relationship to form a first curtain extending from the front wall to a point midway between the front and rear wall, a second vertical header mounted outside and adjacent the rear wall, another series of water tubes con-.

nected to the second header and passing through the rear wall into the furnace, the latter series of tubes extending vertically and parallel to one another within the furnace to form a second curtain extending from the rear wall midway to the front wall, the first and second curtains lying in the same general vertical plane to form a first divider curtain, a second divider curtain similar to the first, the divider curtains serving to divide the furnace into compartments, a first group of burners mounted on the front wall at a position generally midway between the side walls and lying within a central compartment formed by-the divider curtains, a second group of burners mounted on the front wall adjacent one side wall and lying within a side compartment formed bythe divider curtain, a third group of burners mounted on the front'wall adjacent the other side wall and lying within another side compartment formed by a divider curtain, a fourth group of burners underlying the other groups and extending from side wall to side wall, first means furnishing the first group of burners with secondary air, second means independent of the first means for supplying the second, third and fourth groups of burners 'With secondary air, and means for controlling the rate of firing of the first group independently of the second, third and fourth groups to obtain even. gas temperature distribution across the upper portion of the furnace.

4. A steam generating unit comprising a furnace having front, rear and side walls, three forward vertical headers mounted outside the furnace adjacent the front wall, a series of water tubes connected to the headers and passingthrough the front wall into the furnace, the said water tubes extending upwardly within the furnace in parallel relationship to form three forward curtains extending longitudinally of the furnace from the front wall to a point midway between the front and rear wall, three rearward headers mounted outside and adjacent to the rear wall, another series of water tubes connected to the rearward headers and passing through the rear wall into the furnace, the latter series of tubes extending vertically and parallel to one another within the furnace to form three rearward curtains extending longitudinally of the furnace from the rear wall midway to the front wall, the forward and rearward curtains lying in the upper part of the furnace in three vertical planes and serving to divide the furnace into four compartments, a first group of burners mounted on the front wall at a position within the center two compartments, a second group of burners mounted on the front wall within one side compartment, a third group of burners mounted on the front wall within the other side compartment, a fourth group of burners underlying the other groups and the curtains, first means furnishing the first group of burners with secondary air, second means independent of the first means for supplying the second, third and fourth groups of burners with secondary air, and means for controlling the rate of firing of the first group independently of the second, third, and fourth groups to obtain even gas temperature distribution across the upper portion of the furnace.

5. A steam generating unit comprising a furnace having front, rear and side walls, a boiler having a superheater and a reheater, a first vertical header mounted outside the furnace adjacent the front wall, a series of water tubes connected to the header and passing through the front wall into the furnace, the said water tubes extending upwardly within the furnace in parallel relationship to form a first curtain extending from the front wall to a point midway between the front and rear wall, a second vertical header mounted outside and adjacent to the rear wall, another series of water tubes connected to the second header and passing through the rear wall into the furnace, the latter series of tubes extending vertically and parallel to one another within the furnace to form a second curtain extending from the rear wall midway to the front wall, the first and second curtains lying in the same general vertical plane and forming a 'first divider curtain, a second divider curtain similar to the first, the divider curtains serving to divide the furnace into compartments, a first group of burners mounted on the front wall at a position generally midway between the side walls and lying within a central compartment formed by the divider curtains, a second group of burners mounted on the front wall adjacent one side wall and lying within a side compartment formed by a divider curtain, a third group of burners mounted on the front wall adjacent the other side wall and lying within another side compartment formed by a divider curtain, a fourth group of burners underlying the other groups and extending from side wall to side wall, first means furnishing the first group of burners with secondary air, second means independent of the first means for supplying the second, third and fourth groups of burners with secondary air, and means for controlling the rate of firing of the first group independently of the second, third, and fourth groups to obtain even gas temperature distribution across the upper portion of the furnace, a portion of the superheater overlying the second group of burners, another portion of the superheater overlying the third group of burners, a portion of the reheater lying along the rear Wall opposite the first group of burners and being exposed to the radiation of the furnace, the remainder of the reheater overlying the first group of burners and receiving heat by convection.

References Cited in the file of this patent UNITED STATES PATENTS 1,930,566 Sanders Oct. 17, 1933 2,100,190 Jackson Nov. 23, 1937 2,123,860 Blizard July 12, 1938 2,244,144 Drewry June 3, 1941 2,254,226 Koch Sept. 2, 1941 2,285,442 Kerr June 9, 1942 2,293,736 Heller Aug. 25, 1942 

